Fears have lessened that the sinking of the oil rig Deepwater Horizon will cause a major oil spill in the Gulf of Mexico, according to the U.S. Coast Guard, which announced Friday that petroleum had stopped flowing from the seafloor drilling site.

But the incident provides new ammunition for environmentalists girding to fight U.S. government plans to expand offshore drilling.

With hopes for survivors fading, the Coast Guard continued to search Friday for 11 missing Deepwater Horizon crewmembers. Operated by the offshore-drilling company Transocean and leased by BP, the oil rig suffered an unexplained explosion Tuesday and burned until its sinking midday Thursday. The investigation into the incident could take weeks, Transocean said.

A remotely operated submersible is monitoring the deep-sea drilling site both for further oil leakage and for any risk that the Deepwater Horizon's debris might pose to nearby oil pipelines.

"This will go down in the history books as the Earth Day blowout," said Richard Charter, senior policy adviser for marine programs with the conservation group Defenders of Wildlife, referring to the blast that ultimately sank the rig.

The spill caused a slick 2 miles (3.2 kilometers) wide and 8 miles (13 kilometers) long at the last flyover, Petty Officer Atkeson added.

That would make it a "minor" spill, in the view of the U.S. federal response agencies. Spills greater than 2,381 barrels (100,000 gallons/380,000 liters) are characterized as major.

Under current conditions, Atkeson said, the oil slick would take nine days to reach shore—exactly what shore depends on changing currents. But six skimming vessels and a barge, all contracted by BP, are trying to prevent the oil from reaching land.

The oil rig itself had 700,000 gallons (2,650,000 liters) of diesel on board, and as of Friday authorities don't know whether that fuel was still in the rig when it sank or whether the fire consumed the oil product.

Although the flow of oil from the subsea well appears to have stopped, it's not clear why. The Coast Guard on Friday said it was preparing for a worst-case scenario, in case the seal doesn't hold.

"We remain in a 'ready to respond' mode," Rear Adm. Mary Landry, commander of the Eighth Coast Guard District, said in a prepared statement.

The sinking of the Deepwater Horizon came just days before a major climate change bill is set to be unveiled in the U.S. Senate.

On Monday, Congress will see for the first time the long-awaited proposed legislation—a plan to tackle global warming while expanding domestic sources of energy, including from expanded offshore drilling. The moves to lift restrictions on production are designed to garner support from Republicans and conservative Democrats.

Defenders of Wildlife's Charter and other conservationists who've been briefed on the Senate climate change bill say it would go even further in opening the United States' outer continental shelf to energy exploration than the Obama Administration's plan, unveiled last month.

For instance, the bill is said to contain a provision to sidestep environmental study of the impact of seismic prospecting for oil and natural gas on the U.S. coastline. The climate change bill would also have the federal government itself pay for such testing, Charter said.

The Deepwater Horizon incident naturally will now be a backdrop in the coming debate, Charter said. Department of the Interior Secretary Ken Salazar "has consistently said that we can only go forward with this drilling plan if we have enough information," he said.